1. Field of the Invention
This invention relates to a safety device for a nuclear reactor cooling pump.
2. Description of the Related Art
A conventional nuclear power station contains a nuclear reactor, three or four steam generators and three or four cooling circuits for cooling the nuclear reactor in which circulates a primary cooling fluid. In PWR (Pressurized Water Reactor) nuclear power stations, the primary cooling fluid is water, which is placed under a pressure of approximately 15.5 MPa. Each circuit comprises a "hot" branch through which the high-temperature cooling fluid flows from the nuclear reactor to the first expansion chamber of each steam generator. A thermal interchange then occurs between the cooling fluid and the water supplied to each steam generator.
Each circuit also contains a "cold" branch in which the cooling fluid, whose temperature has been lowered by the above-mentioned heat exchange, flows from the second expansion chamber of each steam generator to the nuclear reactor. Finally, the "cold" branch of each circuit is fitted with a primary motor-driven pump which circulates the cooling fluid.
Conventionally, the axis of the primary motor-driven pump units is vertical, with the motor arranged above the pump on a support. The cooling fluid is sucked in at the lower part of the pump, along the pump axis, and is then discharged either laterally, radially or tangentially.
As shown in FIG. 1, pump 1 generally comprises a housing 2 which is terminated at its upper part by a seal housing 4, pump shaft 3 being mounted so as to rotate in this housing 4.
When the motor turns drive shaft 3, the wheel inside pump housing 2 makes the cooling fluid circulate at pressures ranging from atmospheric pressure up to 15.5 MPa and at a temperature in the order of 280.degree. to 300.degree. C. This pressurized cooling fluid exerts an upwardly directed force on the pump shaft as far as the upper part of seal housing 4 is at atmospheric pressure.
Three seals 5, 6 and 7 are generally provided to allow pump shaft 3 to rotate freely in seal housing 4 and ensure sealing between the inside of pump housing 2 and the outside of the seal housing. These three seals 5, 6, 7 are arranged around the shaft, one above the other, inside housing 4.
This set of three successive seals therefore normally ensures the sealing of the shaft. However, if this set of seals either completely or partially fails, the downside (downstream side) elements are subjected to pressures in excess of their design pressures, and if they rupture, the contaminated pressurized fluid spills out and fills the chamber of the reactor building.
This is why a safety device is provided, such as the device referenced 8 in FIG. 1. This device is arranged inside the upper part of seal housing 4.
Such a safety device is commonly called a shut-down sealing device because it is activated after the primary pump has been stopped.
Commonly assigned French Patent No. 80 01 517 (publication No. 2,434,605) describes such a safety device for a primary pump.
This device comprises a hollow cylindrical piston which coaxially surrounds the shaft of the pump and which is located on the downside of third seal 7. This piston is normally located in a housing made for this purpose in the seal housing. In the event of a rupture, the pump is first stopped and then the means provided to move the piston activated, The piston then compresses a seal against the shaft which stops the fluid from flooding the reactor chamber.
A number of drawbacks of this safety device have nevertheless been noted.
First, it was noted that in the event of at least two seals rupturing, the pressure created on the upside upstream side) of the hollow piston became relatively high and then caused the piston to engage with the shaft before the pump had been slowed down and stopped and the means for axially moving this piston activated. As mentioned above, this safety device is only intended to function when the pump is at a stand-still so as to avoid any deterioration. It therefore became necessary in practice to arrange two rods perpendicularly to the shaft to maintain the piston in the lower position. These rods are only released when the means for axially moving the piston are activated.
It can be readily appreciated that the presence of these two rods makes for greater complexity in both the manufacture and operation of the safety device.
The preferred means for moving the piston are formed by an auxiliary pressurized fluid source, compressed air for example, it also being noted that this pressure had to be relatively high, possibly reaching as much as 5 MPa.